1) Some small trimming might be required. I have a 70 coupe. I had to make the two top notches about 1/2" deeper, and had to trim about 3/8" off the driver side wheel well cutout.
2) I doubt this will apply to most, but i have a serious sound system in my cat. This cut down on the sub Hz's getting into the cab. Just about 15% though.
1) Holy crap does this thing stiffen up the chassis twist.
Use a sheet of dynomat if possible. I did and it lowers the resonating frequency of the sheet metal substantially. In my opinion a silicon gasket is not required. After cranking down all the self tapping screws, no rattles were heard.